From June 18th to 22nd, the Penn State Extraterrestrial Intelligence Center (PSETI) held the second annual Penn State SETI Symposium. The event saw experts from many fields and backgrounds gathering to discuss the enduring questions about SETI, the technical challenges of looking for technosignatures, its ethical and moral dimensions, and what some of the latest experiments have revealed. Some very interesting presentations examined what will be possible in the near future and the likelihood that we will find evidence of extraterrestrial intelligence.
Among them, there were some very interesting presentations by Adam Frank, Professor of Astrophysics at the University of Rochester; Ph.D. student Matias Suazo, an astrophysicist and member of Project Haephestos at the University of Uppsala; and Nicholas Siegler, the Chief Technologist of NASA’s Exoplanet Exploration Program (ExEP). These presentations addressed ongoing issues in the search for extraterrestrial intelligence (ETI), technosignatures, the role of oxygen in the evolution of complex life, and what motivations extraterrestrial civilizations (ETC) might have for creating noticeable signatures.
Oxygen and Advanced Life
On Monday, July 19th, as part of the first Plenary Session, Adam Frank delivered a presentation titled “Technosignatures and the Oxygen Bottleneck.” During the talk, Frank whether an oxygen-rich atmosphere at sufficient densities is a precursor to the emergence of complex life and “technospheres,” which Frank described as “the equivalent [of biospheres], the sum total of technological activity on a planet.” These findings were presented in a paper co-authored by Frank and Amedeo Balbi (an associate professor of astrophysics at the University of Rome) that is being reviewed for publication in Nature Astronomy.
Given its importance to the evolution of terrestrial life, oxygen gas is considered a key indicator of potential habitability and life (aka. a biosignature). On Earth, atmospheric oxygen began to appear roughly 2.8 billion years ago with the emergence of single-celled photosynthetic bacteria, which slowly converted carbon dioxide (the main component of Earth’s atmosphere then) into oxygen gas. This led to the Great Oxidation Event (ca. 2.3 billion years ago), followed by the emergence of multi-celled organisms and other complex life forms.
However, modern research has shown that oxygen-rich atmospheres could also result from chemical disassociation, where exposure to solar radiation causes water molecules to split into hydrogen gas and oxygen gas. This process creates atmospheric oxygen that is not the byproduct of organic processes and not indicative of life (“abiotic” oxygen.) In essence, oxygen gas may not be a reliable biosignature, and its role in the emergence of advanced life remains an open question. But citing previous research, Frank argued that all life has limited options (i.e., the periodic table of elements) and how it has evolved to harness different energy sources.
This included how early life harnessed heat energy from hydrothermal vents, cyanobacteria harnessed photosynthesis (solar radiation), and more advanced life (including humans) harnessed fire. This had a profound impact on the evolution of human societies and planet Earth itself (in the form of climate change). But as Frank and Balbi argued, fire is impossible without a sufficiently oxygen-rich environment and at sufficient pressures. Frank illustrated this by showing how combustion is necessary for many of the technological processes humanity has come to depend on.
This includes everything from hunter-gatherers building fires to keep warm at night and in cold climates, metallurgists creating tools from copper, bronze, iron, and steel, demolition experts dynamiting rock and natural barriers, and aerospace engineers launching chemical rockets. According to Frank, these reactions are only possible in an atmosphere where oxygen partial pressures are 18% or higher – P(O2) > 18%. This leads to the inevitable question, “Can a young tool-using, tool-building species develop technosignature-scale technologies without combustion.”
According to Frank and his colleagues, the answer is a resounding “no.” Based on their analysis, atmospheric oxygen is a “bottleneck” where the evolution of advanced life is concerned.
The Hunt for Dyson Spheres
The presentation by Matias Suazo concerning Project Hephaistos was part of Plenary Session Seven (which took place on Wednesday, July 21st). The project is dedicated to searching the Milky Way for signs of Dyson Spheres, the theoretical megastructure proposed in 1960 by Dr. Freeman Dyson (1923-2020). In his seminal paper, “Search for Artificial Stellar Sources of Infrared Radiation,” Dyson proposed how ETI that has reached a high level of technical development might build a megastructure to enclose their entire star system, thus harnessing all of the energy from their parent star.
Dyson also suggested that dedicated SETI surveys could search for these megastructures by looking for indications of waste heat radiated in the far-infrared wavelength. Project Hephaistos, launched by the University of Upsalla in 2015, is Sweden’s first SETI initiative and relies on data from advanced observatories to search for technosignatures associated with large-scale engineering projects rather than the traditional signals-based approach. For his presentation, Suazo summarized the process he and his colleagues at Hephaistos have used to find evidence of mega-scale engineering around other stars.
This includes the possibility that megastructures affect the characteristics of their host galaxies, including reduced optical luminosity, boosted infrared luminosity, and morphological anomalies. To this end, the project scientists have combined data from the ESA’s Gaia Observatory, Two Micron All Sky Survey (2MASS), and NASA’s Wide-field Infrared Survey Explorer (WISE). Using a Sun-like spectrum as a starting point, Suazo shared how the project set an upper limit on temperature (T) and luminosity (L) to look for drops in optical flux and boosts in mid-infrared radiation.
He then described the pipeline established by the project to gather data on stars that “exhibit unusual infrared radiation not easily attributable to known astronomical sources.” This combined astrometry data from Gaia with infrared data from 2MASS and WISE, ranging from the near- to the mid-infrared spectrum (1.25 um to 22 um). This yielded 5 million sources within a volume of space measuring 300 parsecs (~1,000 light-years) in diameter. The next step was isolating sources producing elevated mid-infrared (12 to 22 um) radiation, reducing the candidates’ number to 320,000.
For the third step, Suazo and his colleagues created 220,745 models that considered astrometric data alongside temperature and luminosity to isolate sources that were a “good fit” for Dyson structures. This further reduced the number of candidates to around 11,000. The fourth step was to conduct an image classification scheme using a convolutional neural network that eliminated stars in nebular regions – these produce IR signatures similar to what would theoretically be observed with Dyson structures. This reduced the number of candidates to 5,700 sources.
A final step was to implement “additional cuts,” which prioritized sources that were point-like (stars rather than galaxies or quasars), showed no optical/mid-IR variability, no hydrogen-alpha emissions (typical of protoplanetary disks), and individual stars systems (not binary or multi-star systems). This brought the total to ~4,000 candidates, which they subdivided based on temperature – over or under 200 K (-73.15 °C; -99.67 °F) and “confounding variables” that led to questionable results (and possible false positives). Ultimately, they devised a list of 20 viable candidates for follow-up observations.
The project plans to conduct these observations using optical telescopes, which will help them discard potential false positives (such as debris disks) and narrow the search even further.
Are Aliens Looking for Us?
Another fascinating presentation was conducted by Nick Siegler titled “Alien Motivations and their Technosignature Search Approaches.” The focus of this presentation was an update on the technosignature study the NASA Exoplanet Exploration Program (ExEP) began about 18 months ago (and was talked about at the 2022 PSETI Symposium) that examines how scientific investigators are searching for signs of potential alien civilizations. According to Siegler, the ExEP’s Charter comes down to three main objectives:
- Discover planets around other stars.
- Characterize their properties.
- Identify candidates that could harbor life.
So far, the ExEP has been working towards creating a telescope that can look at any random star in our galaxy and attenuate its on-axis starlight so any exoplanets orbiting them can be seen. This is known as the Direct Imaging Method, where exoplanets are observed and characterized through direct observation rather than the most widely-used indirect methods (Transit Photometry and Doppler Spectroscopy). This can be done using internal and external occulters that block out starlight, otherwise known as coronographs and starshades.
Once this is done, astronomers will obtain spectra from exoplanet atmospheres to discern the presence of biosignatures. But as Seigler noted, these same observations could also stumble upon potential technosignatures like industrial pollutants, an orbital beacon, or other indications of an advanced civilization:
“We realize that, as we’re conducting these biosignature missions, there are opportunities for commensal technosignature research. And so it got us thinking, what else could we do to enhance the commensal relationship we have with technosignatures so we wouldn’t cost NASA a lot of money but be able to do more of these.”
This was the genesis of the ExEP technosignature study, a fact-finding study to inform NASA of where it can positively impact the investigation of technosignatures. This includes a rundown on methodology, which current and future observatories could use to conduct commensal biosignature/technosignature studies, what technology is required, and other necessities (funding, etc.). This study, said Seigler, is about 70% finished and will be made public upon completion. Some of the key details of the report were shared during the Breakout Session held the following day.
But what has been especially interesting about their study, said Seigler, is the question of motivations: what would motivate an extraterrestrial civilization to create noticeable technosignatures? This speculation could help scientists decide what type of technosignatures to investigate, saving time and resources by constraining the search. According to Seigler, the possible motivations they considered include:
- Building megastructures to collect their star’s energy (aka. Dyson structures)
- Calling attention to their existence (radio signals, beacons, IR pulses, transit events)
- Large cities and industries on their planet (pollutants, mid-IR heat, artificial lighting, radio “leakage”)
- Swarms of satellites (Clarke Belts) and space debris (reflected light, transit photometry)
- Signs of terraforming (similarities/abnormalities on multiple planets)
- Megastructures to reduce global warming (solar shades, reflective satellites)
- Indications of spacecraft (exhaust from nuclear fusion, antimatter, radio signals, nebula contrails)
- An interstellar communication network (radio communications)
Another major question Seigler raised was whether or not aliens could be observing Earth, and if so, how would we detect them? This could include reflected light from spacecraft lurking in Lagrange Points or making flybys (he included ‘Oumuamua as a potential example), artifacts from landers, rovers, and other robotic sentries, the detection of radio signals, and deliberate messages they left behind (for example, in the genome of terrestrial organisms). All of these, said Seigler, are viable approaches and could reveal evidence of past visitations.
These presentations offer a good cross-section of the questions and discussions raised at the 2023 Penn State SETI Symposium. They also highlight how far the field of study has come and what will be possible in the coming years. If there’s one takeaway, it is the fact that the Symposium raised more questions than it answered. This is no coincidence, as the field of SETI remains one of the most ambiguous and mysterious scientific pursuits there is. When it comes to the possibility of intelligent life elsewhere in the Universe, we are limited in terms of what we know and how we know it.
Alas, that is the most important reason for us to keep looking. Until the day comes that we find evidence of other civilizations, we’ll never have more than a provincial understanding of what life is, under what circumstances it can emerge, and whether or not life and intelligence are ubiquitous in the Universe.
Further Reading: Penn State SETI Symposium
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